📝 Document matcher caveats in the README

State the end-user limits once, in README § Caveats: a member colliding
with its stringification, the unsupported symbol/bigint, and NaN/-0. Drop
the duplicated known-issue prose from development/library.md and the
source comment; link to the README instead.

development/library.md

@@ -97,7 +97,7 @@ Each factory is two overloads whose order is load-bearing:
 #### Decision (2026-09)
 
 The universe (`Matchable`) is `string | number | boolean | null | undefined`,
-with `boolean` admitted as `true | false`. `symbol` and `bigint` are not.
+with `boolean` admitted as `true | false`.
 
 `boolean`/`null`/`undefined` are not property keys, so handler-map keys are a
 projection (`PatternKey`: each member stringified) and `PatternParam` inverts
@@ -109,18 +109,6 @@ offers `true`, `false`, `null`, `undefined` by name (verified over LSP).
 - Runtime dispatch is unchanged in effect: `handlers[true]` already coerces to
   `"true"`. `dispatch` wraps the index in `String()` only because TypeScript
   forbids indexing with `boolean`/`null`/`undefined` (`TS2538`).
-
-#### Rejected
-
-- **`symbol`.** A brand is a compile-time phantom — nothing to match at
-  runtime; the popup cannot offer symbol keys anyway.
-- **`bigint`.** Not a valid property key; a stringified key (`"1"`) collides
-  with numeric `1`.
-- **`NaN` / `-0`.** No literal type exists; they stay one `number`.
-
-#### Known issue
-
-- A universe mixing a member with its stringification (`1 | "1"`,
-  `true | "true"`, `null | "null"`) collapses to one handler key and routes
-  both members to it. Pre-existing for `1 | "1"`; now reachable for the new
-  members. Not guarded at the type level.
+- `symbol`/`bigint`/`NaN`/`-0` are rejected, and a member colliding with its
+  stringification is unguarded: user-facing, stated once in
+  [README § Caveats](../README.md#caveats).